1. Field of the Invention
The present invention relates to cross-coupled filters, and more particularly, to a cross-coupled bandpass filter for generating a transmission zero in a high frequency rejection band.
2. Description of Related Art
Current portable communication devices have high requirements on pass band selectivity. Trisection cross-coupled bandpass filters are usually used to achieve high selectivity. In general, a microstrip filter using magnetic cross-coupling can generate a transmission zero in a low frequency rejection band while a microstrip filter using electric cross-coupling can generate a transmission zero in high frequency rejection band.
FIG. 1A is a schematic view of a conventional bandpass filter using electric cross-coupling. Referring to FIG. 1A, the conventional bandpass filter has three microstrip open-loop resonators 12, 14, 16 formed on a dielectric substrate 11. FIG. 1B shows the frequency response of the bandpass filter of FIG. 1A. Therein, curve C1 shows that a transmission zero can be generated in a high frequency rejection band. However, during an integrated passive device (IPD) fabrication process, input and output signals are fed through capacitors. As such, the capacitors connected to an input port (open-loop resonator 14) and an output port (open-loop resonator 16), respectively, are too close to each other, thus easily causing short circuits.
FIG. 2A is a schematic view of a conventional bandpass filter using magnetic cross-coupling. Referring to FIG. 2A, the bandpass filter has three microstrip open-loop resonators 22, 24, 26 formed on a dielectric substrate 21. FIG. 2B shows the frequency response of the bandpass filter of FIG. 2A. Therein, curve C2 shows that a transmission zero can be generated in a low frequency rejection band. However, during an IPD fabrication process, it is difficult to use electric cross-coupling to generate a transmission zero in a high frequency rejection band.
FIG. 3A shows a magnetically cross-coupled bandpass filter 30 using an IPD fabrication process. Referring to FIG. 3A, the bandpass filter 30 has a resonator consisting of an inductor 32 and a capacitor 33a, a resonator consisting of an inductor 34 and a capacitor 35a and a resonator consisting of an inductor 36 and a capacitor 37a. The capacitor 35a serves as a signal input port, and the capacitor 37a serves as a signal output port. Two terminals of the inductor 32 constitute an opening 32a, and the two terminals are electrically connected together through the capacitor 33a and a capacitor lower plate 33b. Two terminals of the inductor 34 constitute an opening 34a, and the two terminals are electrically connected together through the capacitor 35a, a capacitor lower polar plate 35b and a through hole 35c. Similarly, two terminals of the inductor 36 constitute an opening 36a and the two terminals are electrically connected through the capacitor 37a, a capacitor lower polar plate 37b and a through hole 37c. FIG. 3B shows the frequency response of the bandpass filter 30. Curve C31 shows that a transmission zero is generated in a low frequency rejection band. Curve C32 shows that input reflection S11 and output reflection S22 are nearly the same in such a symmetrical configuration.
As described above, it is difficult to achieve electric cross-coupling in an IPD fabrication process so as to generate a transmission zero in a high frequency rejection band. Therefore, there is a need to provide a cross-coupled bandpass filter that is applicable in an IPD fabrication process so as to effectively use magnetic cross-coupling to generate a transmission zero in a high frequency rejection band.